1. Field of Invention
The present invention relates generally to the detection of leakage signals and the location of leaks, and more particularly to a low-cost leakage detector and method for detecting digital signals leaking from the coaxial portion of a hybrid fiber-coax (HFC) network and for locating the leaks.
2. Background Art
The detection of digital signals leaking from an HFC network and the location of the leaks are very important tasks for controlling the ingress and egress of the HFC network, such as a cable television network.
Known methods of leakage detection and location in HFC networks have involved the detection of existing analog TV carrier signals or special pilot signal inserted into the network. Such methods may also include some of tag signal or modulation imposed on the analog TV carrier or pilot signal to mark the detected signal as leakage originating from the HFC network. Examples of using an analog TV carrier are disclosed in the following patent documents: Pub. App. No. 2008/0133308 (Jun. 5, 2008) to Harris; Pub. App. No. 2008/0033698 (Feb. 7, 2008) to Stelle; Pub. App. No. 2007/0022457 (Jan. 25, 2007) to Eckenroth et al.; U.S. Pat. No. 7,548,201 to Eckenroth et al.; U.S. Pat. No. 7,395,548 to Runzo; U.S. Pat. No. 6,801,162 to Eckenroth et al.; Pub. App. No. 2006/0248565 (Nov. 2, 2006) to Shimp et al.; U.S. Pat. No. 6,833,859 to Schneider et al.; U.S. Pat. No. 6,313,874 to Bowyer et al.; and U.S. Pat. No. 5,777,662 to Zimmerman. Examples of using an inserted pilot signal are found in the following patents: U.S. Pat. No. 6,600,515 to Bowyer et al.; and U.S. Pat. No. 4,072,899 to Shimp. The use of tag signals in connection with analog TV carriers or pilots signals are disclosed in the following patent documents: U.S. Pat. No. 6,804,826 to Bush et al.; U.S. Pat. No. 6,600,515 to Bowyer et al.; and U.S. Pat. No. 6,018,358 to Bush.
The known methods have limitations in cases where the HFC network contains only digital signals, such as quadrature amplitude modulation (QAM) signals (an “all-digital network”). Many modern all-digital networks do not have analog channels (or analog carriers) and do not have unused bandwidth for test signals or pilot carriers. Current analog detection techniques are not effective for leaks of QAM television signals or other digital TV signals, because such leakage signals look like noise. Thus, if such analog techniques are to be used in an all-digital network, expensive bandwidth will need to be allocated for transmitting analog carriers or pilot signals to be detected as leakage signals. The use of a QAM receiver to detect a QAM leakage signal may initially seem to be a solution, but such receivers usually require a carrier-to-noise ratio (CNR) of better than 20 dB for demodulation. QAM leakage signals typically have a CNR that is significantly lower than 20 dB.
The problem of detecting leakage in an all-digital network is described in the following articles by Ron Hranac: “Broadband: Signal leakage in all-digital network” http://www.cable360.net/ct/operations/bestpractices/33882.html (Feb. 1, 2009) and “Broadband: Signal leakage in all-digital network: Continuing story” http://www.cable360.net/ct/sections/-columns/broadband/35443.html (May 1, 2009). Also the problem is discussed in the publication, “Leakage in all-digital World” http://www.cablefax.com/technology/strategy/-Leakage-in-an-All-Digital-World—34303.html (Mar. 1, 2009).
Copending application Ser. No. 12/583,263, published as U.S. Patent Application Publication No. 2011/0043640 (Feb. 24, 2011), filed by the inventor herein, discloses a system for detecting and locating digital signals (e.g., QAM television signals) leaking from an HFC network. To achieve detection of such low level digital, noise-like signals, and ultimately to locate them, the system employs coherent cross-correlation. Samples of the digital signal carried on the HFC network (“reference samples”) are coherently cross-correlated with samples of the same signal leaking from the HFC network into free-space (“leakage samples”). The reference samples are generated by a headend or reference unit connected to the HFC network and the leakage samples are generated by a leakage detector unit that receives the leaked signal from free-space. Coherent cross-correlation requires synchronization of the reference and leakage samples. Synchronization is achieved by using the clock signal, seconds or sync pulses, and timestamps (collectively, “synchronizing signals”) from a satellite navigation system, such as the Global Positioning System (GPS). The reference and detector units include receivers for receiving the synchronizing signals. Cross-correlation is usually performed in the detector unit, which receives the reference samples via a communicate link such as, e.g., a mobile wireless network connection. The detector unit is usually mounted in a service vehicle which travels along the HFC network to locate the leak.
The system described in copending application Ser. No. 12/583,263 quickly and accurately identifies a particular subscriber premises or network device, or a few candidate network devices, as the source of the leak. If a few candidate devices are identified, it may be desirable to manually search on foot along a street (or up a pole) with the leakage detector to pinpoint or confirm a particular leaking network device or other leak location. If a subscriber's premises is identified as the source of the leak, it may be desirable to manually search inside or around the premises to find the leak. To conduct such manual searches, the leakage detector would have to be removed from the service vehicle. Such removal can be inconvenient and time consuming. It can be inconvenient because the detector must be dismounted from a bracket and disconnected from vehicle-mounted antennas and reconnected to mobile antennas. It can be time consuming because the leakage detector may lose acquisition of the satellite navigation system when switching antennas and it will take time to reacquire the satellite. Moreover, if the detector is to be taken inside a dwelling, it is also likely to lose acquisition of the satellite system, making it difficult or impossible to perform coherent cross-correlation.
One possible solution to the above problem is to carry a second leakage detector in the vehicle, which is already connected to mobile antennas. However, the second detector will still need time to acquire the satellite system and can only be used for an outside application. Further, a duplicate leakage detector and the original each contain a satellite receiver and mobile wireless modem with a paid data plan and each has a not so insignificant overall unit cost. Thus, the use of a duplicate unit is unattractive from a cost standpoint. Accordingly, there exists a need for a more flexible and cost effective solution to extending the reach of the leakage detection system of copending application Ser. No. 12/583,263, for on foot, manual leakage searches along a street, up a pole, or in and around a subscriber's premises.
Recently, there has been an initiative by some cable television system operators to check a subscriber's premises for signal leaks when a technician or installer visits the subscriber to make a repair or install equipment. This initiative is part of an overall maintenance program referred to as “home certification.” In the past, conventional low-cost signal strength meters were used to find analog signal leaks in subscribers' premises. However, cable operators currently offer television programming almost exclusively in digital format, and in particular, in the QAM signal format. Such signal strength meters are not suitable for detecting such digital signals. Thus, the home certification program has generated an urgent need for a low-cost meter that can reliably detect and locate digital TV signal leaks inside a subscriber's premises.